Nanomaterials are materials that include components with nanometer dimensions, for example, where at least one dimension is less than 100 nanometers. Examples of such materials are allotropes of carbon such as nanotubes or other carbon fullerenes and components of carbon char. Carbon black was an early use of nanomaterials in tire manufacturing. Other nanomaterials include inorganic materials such as metal sulfides, metal oxides and organic materials. Because of the small dimensions, nanomaterials often exhibit unique electrical and electrochemical properties and unique energy transport properties. These properties are most pronounced when high surface areas are present and when charge transport mechanisms exist in the nanomaterials.
Some nanomaterials are manufactured using rigorous processing steps that are expensive and commercially unattractive. Some nanomaterials occur naturally or incidentally in commercial processing steps. Naturally or incidentally occurring nanomaterials tend to be highly irregular in size and composition because the environment in which they are produced is not adequately controlled for the production of nanomaterials. Processing methods that produce nanomaterials include among others, liquid-phase steps, gas-phase steps, grinding steps, size-reduction steps and pyrolysis steps.
Pyrolysis is the heating of materials in the absence of oxygen to break down complex matter into simpler molecules and components. When carbon based materials are pyrolyzed, the process of carbonization can occur leading to an ordered state of semi-graphitic material. When carbon based materials are pyrolyzed in uncontrolled conditions, a large amount of randomly ordered carbon material results. When both carbon and inorganic materials are present, pyrolysis under controlled conditions can lead to highly useful and unique results. An example of a use of pyrolysis is for the break down of used tires (typically from automobiles, trucks and other vehicles). The pyrolysis of tires results in, among other things, a carbon/inorganic residue called char.
The composition of char from tire pyrolysis is determined by the materials that are used to manufacture tires. The principal materials used to manufacture tires include rubber (natural and synthetic), carbon black (to give strength and abrasion resistance), sulfur (to cross-link the rubber molecules in a heating process known as vulcanization), accelerator metal oxides (to speed up vulcanization), activation inorganic oxides (principally zinc oxide, to assist the vulcanization), antioxidant oxides (to prevent sidewall cracking), a textile fabric (to reinforce the carcass of the tire) and steel belts for strength. The carbon black has a number of carbon structures including graphitic spheroids with nanometer dimensions, semi graphitic particles and other forms of ordered carbon structures.
In summary, the manufacture of tires initially mixes the materials to form a “green” tire where the carbons and oxides form a homogenous mixture. The “green” tire is transformed into a finished tire by the curing process (vulcanization) where heat and pressure are applied to the “green” tire for a prescribed “cure” time. The carbon materials used in “green” tires are typically as indicated in TABLE 1:
TABLE 1DESIGNATIONSIZE (nm)N11020-25N22024-33N33028-36N30030-35N55039-55N68349-73
When tires are discarded, they are collected for pyrolysis processing to reclaim useful components of the tires. In general, tire pyrolysis involves the thermal degradation of the tires in the absence of oxygen. Tire pyrolysis has been used to convert tires into value-added products such as pyrolytic gas (pyro-gas), oils, char and steel. Pyrolysis is performed with low emissions and other steps that do not have an adverse impact on the environment. The basic pyrolysis process involves the heating of tires in the absence of oxygen. To enhance value, the oils and char typically under go additional processes to provide improved products.
The electron transfer can occur at an electrode through the release of chemical energy to create an internal voltage or through the application of an external voltage. Such electrochemical reactions where electrons are transferred between atoms or molecules are called oxidation/reduction or redox reactions. Oxidation and reduction reactions can be separated in space and time and devices with such reactions are often connected to external electric circuits. The creation of internal voltages at electrodes is useful in batteries and the application of external voltages to electrodes is useful in capacitors. In connection with electrochemical reactions at electrodes, the atom or molecule which loses electrons is oxidized, and the material which accepts the electrons is reduced.
In battery cells, electric current is generated from energy released by a spontaneous redox reaction. The battery cells have two electrodes (the anode and the cathode). The anode is the electrode where oxidation occurs and the cathode is the electrode where reduction occurs.
The electrodes of a battery cell are in an electrolyte where the cations are the oxidized form of the electrode metal. The tendency of the electrode metals to oxidize or reduce, in a particular electrolyte, is controlled by the electrochemical potential which depends on the temperature, pressure, the composition and concentration of the electrolyte and the nature and composition of the anode and the cathode. In a battery cell, when the anode undergoes oxidation and the cathode undergoes reduction, the sum (sign and magnitude) of the electrochemical potentials at both electrodes produces an electrical potential difference between the two electrodes.
Primary batteries are batteries that are not recharged and are discarded after discharge. Secondary batteries are batteries that are recharged, that is, they are recharged after a discharge and are reused multiple times. There are many known batteries including the following common examples. Each battery type has unique cost and performance advantages and disadvantages.
Lithium Ion batteries are found in consumer electronics including laptops, digital cameras and cell phones. Nickel-cadmium and Nickel-metal hydride batteries are used for rechargeable applications. Alkaline batteries are used for disposable applications. Lead acid batteries have deep cycles and are used in automobiles. One criteria in judging the quality of a battery is its power and energy to weight ratio. While bigger batteries are able to provide more energy, they often do not meet the size requirements in consumer electronics.
Zinc-air batteries (non-rechargeable) and zinc-air fuel cells, (mechanically-rechargeable) are electrochemical batteries powered by the oxidation of zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce. They are used in hearing aids and in experimental electric vehicles. Particles of zinc are mixed with an electrolyte (usually potassium hydroxide solution); water and oxygen from the air react at the cathode and form hydroxyls which migrate into the zinc paste and form zinc oxide hydroxide, ZnO(OH)42−, at which point electrons are released and travel to the cathode. The zinc decays into zinc oxide and water is released back into the system. The water and hydroxyls from the anode are recycled at the cathode, so the water serves only as a catalyst. The reactions produce a maximum voltage level of 1.65 volts. The nickel-cadmium battery (NiCd) is a rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes. Nickel cadmium batteries tolerate deep discharge for long periods in contrast, for example, to lithium ion batteries, which are highly volatile and are permanently damaged if discharged below a minimum voltage. The NiCd batteries have a higher number of charge/discharge cycles than other rechargeable batteries and have faster charge and discharge rates than lead-acid batteries.
Lead-acid batteries are less expensive alternative to NiCd batteries although NiCd batteries are smaller and lighter than comparable lead-acid batteries.
Alkaline batteries have a higher capacity than equivalent NiCd batteries. However, an alkaline battery's chemical reaction is typically not reversible so that a reusable NiCd battery has a significantly longer total lifetime. Since an alkaline battery's voltage drops as the charge drops, most consumer applications are well equipped to deal with the slightly lower NiCd voltage with no noticeable loss of performance.
Nickel metal hydride (NiMH) batteries have a higher capacity and are less toxic than NiCd batteries. NiCd batteries have a lower self-discharge rate (for example, 20% per month for a NiCd, versus 30% per month for a NiMH). This results in a preference for NiCd over NiMH in applications where the current draw on the battery is lower than the battery's own self-discharge rate (for example, television remote controls).
A zinc-carbon battery is typically packaged in a zinc can that serves as both a container and anode. The cathode is a mixture of manganese dioxide and carbon powder. The electrolyte is a paste of zinc chloride and ammonium chloride dissolved in water. Carbon-zinc batteries are low-cost primary batteries. The container of the zinc-carbon battery is a zinc can. The battery contains a layer of NH4Cl with ZnCl2 aqueous paste separated by a paper layer from a mixture of powdered carbon and manganese oxide (MnO2) which is packed around a carbon rod. The outer zinc container is the anode (−). The zinc is oxidized according to the following half-equation.Zn(s)→Zn2+(aq)+2e−
A rod surrounded by a powder containing manganese oxide is the cathode(+). The manganese dioxide is mixed with carbon powder to increase the conductivity of the cathode mixture. The cathode reaction is as follows:2MnO2(s)+2H+(aq)+2e−→Mn2O3(s)+H2O(l)
The H+ comes from the NH4+(aq):NH4+(aq)→H+(aq)+NH3(aq)
and the NH3 combines with the Zn2+. In this half-reaction, the manganese is reduced from an oxidation state of (+4) to (+3). The overall reaction in a zinc-carbon cell can be represented as:Zn(s)+2MnO2(s)+2NH4+(aq)→Mn2O3(s)+Zn(NH3)22+(aq)
The zinc-carbon battery has an open cell voltage of about 1.5 V. The approximate nature of the voltage is related to the complexity of the cathode reaction. The anode (zinc) reaction is comparatively simple with a known potential. Side reactions and depletion of the active chemicals increases the internal resistance of the AAA battery and this causes the cell voltage to drop.
Advances are being made in battery technology research using nanomaterials. In one example, batteries are printed onto a surface with “nanotube ink” using the same zinc-carbon chemistry as ordinary non-rechargeable batteries. The nanomaterial batteries are less than a millimeter thick, are made from two layers containing carbon nanomaterials and have a third layer of zinc foil. The carbon nanomaterials are packed into these layers and form randomly oriented nanomaterial networks that conduct charge. Although use of nanomaterials is promising, the processing has not yet resulted in practical applications.
While batteries of many types are known such as the examples described above, there is a need for improved electrodes based on nanomaterials and for new batteries using the new nanomaterials.